Position vs Runout GD&T Applications

[@pablov9212]

Appreciate you putting the time and effort into this content, as thankless as it might be. Im a machinist that is about to finish a mech eng degree, and I'm shocked at how little GD&T is taught. Even big firms rarely put out fully dimensioned drawings.

[@GeoTolPro]

Glad it helped. Yes, its troublesome that 4 year universities don't dedicate the time to teach tolerancing. Maybe mechanical engineering is too broad of a degree for this specialized topic.

[@hakantorstensson8053]

Greate video! Clear explanation and real application of runout and position for different cases!

[@vanzz3863]

Agreed!

[@jesusreignonhigh6732]

Do you guys have apps for all your guys contents?

[@alexsalchemy]

This is brilliantly explained, thank you!

[@slavikmarinovski2249]

Great video. May i ask; for a round part like an inspection pin, can you specify 1 (perpendicar) face as A and the opppsite face as runout controlled to A? Or does the runout must refer to a datum AXIS?

[@GeoTolPro]

Runout requires reference to a datum axis. Runout on a 90 degree face to the datum axis is the same as perpendicularity.

[@sem7207]

Thanks for the awesome video. I really like how you explain it in terms size-form-orientation-location and how the two characteristics differ in how they take care of them. I got a question about the position tolerance. In your examples in this video and actually anywhere where it's explained, position is shown controlling cylindrical features (or tabs/slots). But what if the feature was a tapered one, such as a cone? I guess runout or even total runout would still work, but would a position tolerance still be OK, just like for a cylinders?

[@GeoTolPro]

Oh, good question. Position is for features of size. Position on "regular features of size" (hole, slot tab, pin, sphere) is well-defined in Y14.5. Irregular features of size (weird closed shapes) is supported with MMC and LMC modifiers because it establishes a virtual boundary that the feature must not violate. There are no examples of position RFS on a conical feature. It maybe could be used by "extension of principle" but not fully supported. The axis of the cone could be established by its unrelated actual mating envelope. I have used position tolerance on plastic holes which are technically drafted cones. Two profile tolerances, one relative to datums (to control location) and one relative to itself (to control size and shape) would be the safe bet on a conical feature.

[@sem7207]

​@@GeoTolPro Thanks for the detailed answer! I have a couple of follow-up questions: First question - you mentioned that the cone axis can be established by its unrelated actual mating envelope. The actual mating envelope as I know it from the standard is well defined for regular FOS such as cylinders; it expands until it is inscribed by a hole or contracts until it is circumscribed by a pin. However, if we have a cone, what is the equivalent of that contraction/expansion? If we think of the AME as a theoretical infinite envelope it has an apex (zero diameter) on one end and an infinite diameter on the other end, so I guess it won't really expand or contract. Maybe the extension of the principle is that the cone angle changes for the envelope so that it is able to contact as many high points as possible on the actual feature? Second question - If to avoid a grey area we define surface profile for the cone as you suggested, with one line related to datums for location and orientation and the other without references for form (and "size"?), then I think we are necessarily tightening the form (and size?) more than the location and orientation (otherwise the second line would be redundant). But what if we care much less about the form (and size?) of the cone than we do about its location? Thanks again!

[@GeoTolPro]

Yes, you have articulated nicely why cones are weird and the expansion/contact with the mating envelope is difficult to describe. I think the angle should remain basic on the unrelated envelope as it contacts the conical feature. And yes, the dual profile frames (one relative to datums and one to itself) controls the size and form tighter than the location. You asked "what if we care much less about the form and size than you do its location". That sounds like a job for position but like we've talked about, position for cones is a little underbaked right now in Y14.5. As long as you're okay with someone filling in the blanks as needed. Dynamic profile modifier can unlock size requirement, but you're still controlling form, orientation, and location tight.

[@sem7207]

@@GeoTolPro Thanks for another clear and interesting answer!!!🙂👍 However, I'm not sure how the dynamic profile can be useful for a cone. Since even a "static" the tolerance can extend as much as necessary within the infinite range from the apex of the basically defined true profile cone to the direction in which the diameter grows, to cover the actual as produced cone, doesn't this already eliminate any local size (or diameter value) control anyway🤔?

[@GeoTolPro]

Hmmm, interesting. I think you're saying a profile with no datum references on a cone is naturally a dynamic profile. It cant control size. Another trick I have used on plastic drafted holes is to control the circular size "at depth shown" from a known datum. This is shown in the casting/molded parts standard (Y14.8). Then you could profile the conical feature with no datum references.

[@bronco7269]

Love this explanation, this same topic came up for us in a recent application. Helps to hear it explained from a pro with easy to understand examples. Would love to see a similar video comparing the application of perpendicularity to surface profile. Thinking it’s a similar analogy where the surface profile has an additional form control. One question I had on this video is why not just tighten the size tolerance instead of having an extra form control? Thank you Scott

[@GeoTolPro]

Glad you liked it. Here is a video explaining the difference between parallelism and profile: kzbin.info/www/bejne/lZaYcoOHgZ17esk This video is close to your request for perpendicularity vs profile. Profile controls location (and orientation and form) where perp and parallelism controls orientation (and form).

[@GeoTolPro]

You could tighten the size to get your form control, but then the part is harder to pass inspection. On a lathe part, its easy to hold form tolerance (circularity/cylindricity) but maybe harder to get the right size. Our ultimate goal with a tolerancing scheme is to guarantee function while allowing maximum manufacturing tolerance.

[@saurabhroy7100]

So if I control the overall profile of the surface then also it will control almost every thing like form, profile, orientation right? Please reply me

[@GeoTolPro]

Yes, profile controls all variation at once (size, form, orientation, and coaxiality)

[@longliu2901]

THANKS